The present invention relates to the field of voltage-controlled oscillators (VCOs). More particularly, the present invention relates to the field of modulation of the operating frequency of a voltage controlled oscillator.
A voltage-controlled oscillator (VCO) forms a periodic output signal where a frequency of the periodic output signal is related to the level of an input control voltage. The center frequency of a VCO is defined as the frequency of the periodic output signal formed by the VCO when the input control voltage is set to a nominal level, such as zero volts. The input control voltage is then adjusted up or down to control the frequency of the periodic output signal.
VCOs are used in variety of applications, such as for transmitting information according to frequency modulation techniques. To implement frequency modulation using a VCO, the center frequency is set equal to a carrier frequency to be utilized for transmitting the information. The input control voltage is then adjusted up or down in response to an information-carrying signal, thus, forming a frequency modulated signal. The frequency modulated signal is transmitted via a transmission medium to a receiver tuned to the carrier frequency. The information-carrying signal is then recovered by demodulating the received signal.
FIG. 1 illustrates a schematic diagram of a conventional VCO including a cross-coupled transistor pair. A first terminal of an inductor L1 and a first terminal of an inductor L2 are coupled to a supply voltage node VCC. A second terminal of the inductor L1 forms a node N1 and is coupled to a collector of a transistor Q1, to a base of a transistor Q2, to a cathode of a varactor diode D1, and to a cathode of a varactor diode D2. A second terminal of the inductor L2 forms a node N2 and is coupled to a collector of the transistor Q2, to a base of the transistor Q1, to a cathode of a varactor diode D3, and to a cathode of a varactor diode D4.
An anode of the varactor D1 and an anode of the varactor D3 are coupled to a first terminal of a resistor R1. A second terminal of the resistor R1 is coupled to receive a tuning control voltage Vtune1. An anode of the varactor D2 and an anode of the varactor D4 are coupled to a first terminal of a resistor R2. A second terminal of the resistor R2 is coupled to receive a modulation control voltage Vmod1. An emitter of the transistor Q1 and an emitter of the transistor Q2 are coupled to a first terminal of a resistor R3. A second terminal of the resistor R3 is coupled to the ground node.
In operation, an output voltage signal Vout1 formed across the nodes N1 and N2 is generally a sinusoid which oscillates at the resonant frequency of the VCO. When the node N1 is at a higher voltage level than the level of the node N2, the transistor Q2 has a higher bias voltage than the transistor Q1. Accordingly, nearly all of the current through the resistor R3 passes through the right side of the VCO (through the inductor L2 and the transistor Q2). This tends to reinforce the voltage at the node N1 being higher than the voltage at the node N2. Accordingly, this results in positive feedback in the VCO.
Eventually, however, because there is little or no current passing through the inductor L1 and the transistor Q1, the voltage at the node N2 tends to rise relative to the level at the node N1. In response, the bias on the transistor Q1 increases while the bias on the transistor Q2 decreases. This reduces the current in the right side of the VCO and increases the current in the left side (through the inductor L1 and the transistor Q1). Eventually, nearly all of the current through the resistor R3 passes through the left side which reinforces the voltage at the node N2 being higher than the voltage at the node N1, through positive feedback.
Because there is little or no current passing through the right side of the VCO, the voltage at the node N1 tends to rise relative to the level at the node N2. In response, the bias on the transistor Q2 increases while the bias on the transistor Q1 decreases. Accordingly, the above-described cycle repeats. In this manner, current is alternately steered through the right and left sides of the VCO, thereby forming a the output sinusoidal signal Vout1 across the nodes N1 and N2.
The tuning control voltage Vtune1 is typically adjusted such that the output periodic signal Vout1 oscillates at the desired center frequency when the modulation control voltage Vmod1 is at a nominal level. The modulation control voltage Vmod1 is then adjusted up or down to control the frequency of the output periodic signal Vout1. A drawback to the conventional VCO illustrated in FIG. 1 is that the frequency deviation obtained in the output signal Vout1 depends upon capacitance of each of the varactors D1-D4 and the amplitude of the modulation control voltage Vmod1. As the capacitance of the varactors D1 and D3 increases in response to adjusting the tuning control voltage Vtune1, the frequency deviation obtained at the output signal Vout1 for a given modulation control voltage Vmod1 level is reduced. Conversely, as the capacitance of the varactors D1 and D3 is reduced in response to adjusting the tuning control voltage Vtune1, the frequency deviation obtained at the output Vout1 for a given modulation control voltage Vmod1 level increases. As a result, the VCO exhibits undesired, non-linear behavior.
FIG. 2 illustrates an exemplary graph of tuning control voltage Vtune1 vs. frequency deviation in the output Vout1 in response to changes in the modulation control voltage Vmod1 for the VCO illustrated in FIG. 1. Thus, when the tuning control voltage Vtune1 is at a level given on the x-axis, and the frequency deviation which results from an incremental (e.g., one millivolt) change in the modulation control voltage Vmod1 is given on the y-axis. As can be seen from FIG. 2, the frequency deviation changes in a non-linear fashion with changes in the tuning control voltage Vtune1. Difficulties can be encountered when attempting to demodulate a signal which has been modulated by a VCO which exhibits such a non-linear characteristic.
Therefore, what is needed is a technique for obtaining a more linear relationship between a tuning control voltage and frequency deviation in a VCO output signal resulting from changes in a modulation control voltage applied to the VCO.
The invention is a method and apparatus for modulation of a voltage-controlled oscillator (VCO). The VCO receives a tuning control voltage for adjusting a center frequency of an output periodic signal formed by the VCO. In addition, the VCO receives a modulation control voltage for modulating the output periodic signal by a content-carrying signal according to frequency modulation techniques. A frequency deviation obtained in the output periodic signal in response to changes in the modulation control voltage is linearized by forming the modulation control voltage as the result of a linear correction polynomial. The linear correction polynomial is preferably of the form:
Vmod2=K0+(K1)(Vtune2)+m(Vsig)
where Vmod2 is the modulation control voltage, Vtune2 is the tuning control voltage, Vsig is the content carrying signal and K0, K1 and m are constants. Appropriate values for the constants K0, and K1 can be determined by measuring the voltage level for Vmod2 required to obtain a desired frequency deviation at various values of the tuning control voltage Vtune2 (e.g., two endpoints and a center value) and with an assumed value of m(Vsig). Such measurements produce a numerical function which can be approximated with an interpolating polynomial by selecting the values for K0 and K1. The value of m can be selected to achieve a desired proportionality between frequency deviation and amplitude of the content-carrying signal Vsig.
A linear correction circuit forms the modulation control voltage. More particularly, a first amplifier having a gain of K1 receives the tuning control voltage (Vtune2). A first summing block then receives the output of the first amplifier and adds the constant K0. A second amplifier having a gain of m receives the content-carrying signal (Vsig). An output of the second amplifier and an output of the first summing block are summed by a second summing block. An output of the second summing block forms the modulation control voltage (Vmod2).
The present invention results in a more linear relationship, in comparison to prior techniques, between a tuning control voltage and frequency deviation in a VCO output signal resulting from changes in a modulation control voltage applied to the VCO.